1. Field of the Invention
This disclosure relates to a signal processing device and a measuring method.
2. Description of the Related Art
A transistor in which an oxide semiconductor is used to achieve an extremely low off-state current is disclosed. The off-state current cannot be sufficiently accurately measured by a generally used current measuring instrument. In view of this, for example, Patent Document 1 and Non-Patent Document 1 each disclose a method in which a circuit in which a gate of a transistor for detection is connected to one of a source and a drain of a transistor whose off-state current is to be measured is formed, a change in potential of the one of the source and the drain of the transistor is obtained from a change in resistance value of the transistor for detection, and the off-state current is obtained from the obtained change in potential and a capacitance value of the one of the source and the drain of the transistor.